What pressure \( \text{(bar)} \) of \( \mathrm{H_2} \) would be required to make emf of hydrogen electrode zero in pure water at \( 25^\circ \mathrm{C} \)?
\( 1 \)
\( 0.5 \)
\( 10^{-7} \)
\( 10^{-14} \)